This invention relates to the field of cranes and more particularly to the control systems and methods for controlling payload pendulation associated with motion of suspended payloads using cranes mounted with mobile platforms.
Cranes are used in virtually any large-scale construction or cargo transportation operation. As an example, the commercial shipping industry has been moving toward high speed non-self-sustaining container ships which do not have onboard cranes. In general, large pier-side cranes are use to load and unload the ships. Disaster relief operations, as well as military operations, have the problem of offloading and onloading container ships and moving supplies ashore in the absence of port facilities. Cranes mounted with a dedicated crane ship can transfer cargo from container ships to small landing craft for transport to shore. In a more difficult cargo transportation example, ships can be replenished while at sea.
During ship offloading and onloading operations, environments lacking a protective harbor subject crane ships to wave motion which can result in motion of the ship which in turn can excite pendulation of a hoisted container. Damage to personnel, cargo, and the participating ships can occur if the payload undergoes excessive pendulation.
In a typical payload transfer maneuver, a crane operator uses translation, rotation, and lifting operations. Often, inexperienced operators must perform transfers at rates sufficiently slow in order to reduce unwanted payload pendulation. Unfortunately, slow crane maneuvers can increase the cost and time involved to move cargo. Additionally, if platform motion and/or wind exists, workers must use tag-lines to steady the cargo, further increasing cost and time. If these disturbances are significant, cargo operations must stop for safety reasons.
One category of cranes consists of overhead gantry cranes. A second category of cranes consists of rotary cranes, of which there are two types: rotary jib cranes and rotary boom cranes. The primary crane differences, from a kinematics viewpoint, are the number of motion degrees-of-freedom (DOF), the type of motion provided: prismatic motion or rotational motion, and the relative connection via substantially rigid links.
An overhead gantry crane incorporates a trolley which can translate in one or two directions in a horizontal plane. Attached to the trolley is a load-line for payload attachment, which can have varying load-line length. Overhead gantry cranes are suitable for construction and transportation applications where the physical environment supports the crane""s required physical overhead structure. Gantry cranes can have three translational motion degrees-of-freedom: two directions of trolley translation and one vertical translation of load-line length (for example, left-right, forward-backward, and up-down translations). Overhead gantry cranes generally have this structure where the primary DOF for end-effector motion are prismatic (i.e., translational) and oriented at right angles.
A rotary jib crane incorporates a trolley which can move along a horizontal jib, which in turn is attached to a rotatable vertical column attached to a crane base. Rotary jib cranes can have three degrees-of-freedom. The first is a column rotation about a vertical axis at the crane base, such that a load-line attachment point undergoes rotation. The second is a horizontal translation of the trolley along the horizontal-fixed-elevation jib, as in a gantry crane. The third is a variable load-line length, also a translation. Rotary jib cranes generally have this structure where the primary DOF for end-effector motion are one rotary joint followed by two prismatic joints.
A rotary boom crane configuration can have a crane column horizontally rotatable about a vertical axis, a luffing boom attached to the column, and a pendulum-like flexible-link attached to the distal end of the boom. A rotary boom crane can have one translation degree of freedom (variable flexible-link length in hoisting) and two rotation degrees of freedom: rotation about the crane column (slewing) and boom elevation through a vertical angle (luffing). Positioning of a payload that pendulates from the flexible-link is accomplished through luff, slew, and hoist commands. Because of kinematic differences between a rotary boom crane and a rotary jib crane, a rotary boom crane configuration has different payload dynamics from a rotary jib crane.
When a hoisted payload is disturbed, the payload and load-line move like a spherical pendulum about the load-line to manipulator attachment point. As an example, a payload moved by a rotary boom crane can be described by two oscillatory degrees of freedom. The first is payload pendulation tangential to an arc traced by the distal end of the boom while slewing the crane (or equivalently, a motion tangential to the column axis of rotation). The second is a payload pendulation radial to the column axis of rotation. Both radial and tangential pendulation are defined as having a zero value when the flexible-link is parallel to a gravitational vector. At the end of a typical point-to-point maneuver, the payload can oscillate in both the radial and tangential directions. The degree of pendulation is dependent on the specific maneuver. The yaw of the payload relative to the flexible-link can be important in some applications.
Pendulation or sway control has been disclosed for overhead gantry cranes and for rotary cranes in stationary environments.
Feddema et al., U.S. Pat. No. 5,785,191 (1998), is an example of operator control systems and methods for pendulation-free motion in gantry-style cranes. Feddema et al. discloses use of an infinite impulse response filter and a proportional-integral feedback controller to dampen payload pendulation in a crane having a trolley moveable in a horizontal plane and having a payload suspended by variable-length flexible-link for payload movement in a vertical plane. Feddema teaches the use of filters and feedback controllers to remove operator-induced pendulation and to dampen residual pendulation in gantry cranes in a stationary environment. Feddema does not teach compensation for payload pendulation due to motion of a platform with which the crane is mounted.
Robinett et al, U.S. Pat. No. 5,908,122 (1999), is an example of a pendulation control method and system for rotary jib cranes. Robinett et al. discloses use of an input shaping filter to reduce pendulation of rotary jib crane payloads during operator commanded maneuvers or computer-controlled maneuvers. Robinett teaches the use of input shaping filters to remove payload pendulation induced by commands to rotary jib cranes mounted with a stationary platform. Robinett does not teach anything about reduction of unwanted payload pendulation due to motion of a platform with which the crane is mounted.
Parker et al, xe2x80x9cOperator in-the-loop Control of Rotary Cranes,xe2x80x9d Proceedings of the SPIE Symposium on Smart Structures and Materials, Industrial Applications of Smart Structures Technologies, San Diego, Calif. Vol. 2721, pp. 364-372, Feb. 27-29, 1996, teaches the use of command shaping filters to remove payload pendulation induced by operator commands to rotary jib cranes in a stationary environment. Parker does not teach anything about reduction of unwanted payload pendulation due to motion of a platform with which the crane is mounted.
Lewis et al., xe2x80x9cCommand Shaping Control of a Operator-in-the-Loop Boom Crane,xe2x80x9d Proceedings of the 1998 American Control Conference, June 24-26, 1998, incorporated herein by reference, is an example of a command shaping control method for rotary boom cranes. Lewis et al. discloses a method of filtering pendulation frequency using an adaptive forward path command shaping filter to reduce payload pendulation in a rotary boom crane. Lewis teaches the use of command shaping filters to remove payload pendulation induced by operator commands to rotary boom cranes in a stationary environment. Lewis does not teach anything about reduction of unwanted payload pendulation due to motion of a platform with which the crane is mounted.
The control systems and methods discussed above teach removal of operator-induced payload pendulation in environments where a crane base is not subject to motion as in an oscillatory environment. Control of command-induced payload pendulation depends on the kinematics of the crane. Motion of a platform with which a crane is mounted also can induce payload pendulation. The control systems and methods discussed above do not teach reduction of unwanted payload pendulation due to motion of the platform with which the crane is mounted.
Overton, U.S. Pat. No. 5,526,946 (1996), is an example of an anti-pendulation control method for level-beam, cantilever cranes, such as gantry cranes and overhead-transport devices. Overton teaches use of a double-pulse approach with precisely-timed acceleration pulses to control a trolley to reduce operator-induced pendulation and to damp pendulation due to external disturbances. Overton does not teach isolation of payload and flexible link from platform motion.
Overton, U.S. Pat. No. 5,961,563 (1999), hereinafter referred to as Overton""99, is an example of anti-pendulation control method for rotating boom cranes. Overton""99 teaches use of a double-pulse approach with precisely-timed acceleration pulses to control a crane to reduce operator-induced pendulation and to damp pendulation due to external disturbances. Overton does not teach isolation of payload and flexible link from platform motion.
Accordingly, there is an unmet need to isolate the payload and flexible link from platform motion throughout a desired payload motion.
The present invention isolates the payload and flexible link from platform motion throughout a desired payload motion. The present invention comprises a control system and method for generating crane commands from a desired payload motion for substantially pendulation-free actual payload motion, wherein the crane is mounted with a mobile platform. This control system comprises a sensor system and a control computer for generating crane commands corresponding to the desired payload motion, adapted to maintain the payload at a defined configuration relative to an inertial frame.
The present invention comprises a platform motion sensor, indicative of a base platform motion relative to an inertial frame, and a motion compensator, responsive to the platform motion sensor, generating crane commands to maintain a crane payload substantially in a defined configuration relative to the inertial frame and indicative of the desired payload motion. In the defined payload configuration, a crane flexible-link is substantially parallel to a gravity vector.
The present invention can further comprise a pendulation damper controller, responsive to a payload configuration sensor, determining an amount of pendulation from a difference between a current actual payload configuration and the defined payload configuration and driving the crane to reduce the amount of pendulation. The present invention can further comprise a command shaping filter, adapted to generate a defined payload configuration by filtering out a residual payload pendulation frequency of the crane from the desired payload motion.
A method according to the present invention generates crane commands to achieve a desired payload motion by compensating for platform motion to maintain a crane payload in a defined configuration relative to an inertial frame. The present invention can further comprise damping payload pendulation. The present invention can further comprise filtering out a residual payload pendulation frequency from the desired payload motion.